Various fuel cell types, defined by their electrolyte, have particular design requirements. In a proton exchange membrane (PEM) fuel cell, one requirement is to provide an effective water management system. A PEM fuel cell includes a membrane confined between respective porous cathode and anode electrodes. These electrodes include the combination of a relatively thin catalyst layer and a porous support plate where the catalyst layer may be deposited either on respective major surfaces of the proton exchange membrane or on the porous support plate. In general, fuel cells function by supplying a gaseous fuel and oxidant to the anode electrode and cathode electrode, respectively. These supply means for the fuel and the oxidant gas distribute the same as uniformly as possible over the catalyzed surfaces of the respective electrodes. In a PEM fuel cell, the electrochemical reaction occurring at the electrodes, when the fuel cell operates, results in electrons and hydrogen ions being formed at the anode. The electrons flow through an external load circuit and the hydrogen ions flow through the membrane to the cathode where they react with oxygen to form water and also release thermal energy. Fuel cells also have a coolant path for guiding water near the electrodes in the cell stack to prevent the fuel cell from overheating.
Preferably, the loss of water evaporated from the cell and removed through the process vents is balanced by the production of water as a by-product of the chemical reaction taking place within the cell stack less that required for fuel processing. Water balance permits water to be recycled in a closed-loop system, thereby avoiding the need and expense to replenish water in the fuel cell system. Maintaining water balance or self-sufficiency is important when powering, for example, automobiles which are not in constant contact with an external water source for replenishment.
As an example, a gasoline powered ambient pressure PEM fuel cell power plant is a fully integrated system that must be self-sufficient in water to be viable. Self-sufficiency means that enough of the water that is formed as a result of the chemical reaction in the cell must be recovered by the system to provide the water that is required to convert the gasoline to hydrogen in the fuel processing system. Water is produced within the cell as a result of the electrochemical reaction and is removed from the cell as a liquid or vapor by well known means. The water vapor in the exit gas streams is partially recovered by passing the air exhaust vent through a condenser to cool the air exhaust vent resulting in the formation of condensate. The condensate is recovered, accumulated and fed to the fuel processing system as required. There are ambient temperature conditions where a PEM fuel cell with single-pass airflow does not recover enough water to be self-sufficient.
For proton exchange membrane (PEM) fuel cells, a drawback to a self-contained water management system is that using relatively dry ambient air as one of the reactants tends to cause a greater loss of water than is generated by the reaction between hydrogen and oxygen in the reactants when operating at high ambient temperatures. Further, excessive loss of water at high ambient operating temperatures can dry out and permanently damage the membranes of the cell stack.
In response to the foregoing, it is an object of the present invention to provide a PEM fuel cell system which overcomes the drawbacks and disadvantages of prior PEM fuel cell systems in maintaining water balance at high ambient temperatures. The above and other objects and advantages of this invention will become more readily apparent when the following description is read in conjunction with the accompanying drawings.